In Long Term Evolution (LTE) standardized in 3rd Generation Partnership Project (3GPP) as the 3.9th generation radio transmission scheme, in order for spectrum efficiency to be significantly improved from the 3rd generation radio transmission scheme, a Multiple Input Multiple Output (MIMO) technology in which a plurality of transmission/reception antennas are used to perform radio transmission is specified. With a spatial multiplexing (SM) technology that is one of the MIMO technologies, it is possible to realize the enhancement of a transmission rate without increasing a frequency bandwidth. LTE-Advanced (LTE-A) that is an advanced version of LTE is approved as one of the 4th generation radio transmission schemes by International Telecommunication Union, Radio communication Sector (ITU-R), and its standardization activity is briskly performed. In LTE-A, in order to achieve a peak transmission rate of 1G bps in a downlink (base station apparatus→mobile station apparatus) transmission, a single user MIMO (SU-MIMO) that can spatially multiplex a maximum of 8 streams is examined. SU-MIMO is MIMO transmission between a base station apparatus having a plurality of transmission antennas and a single mobile station apparatus having a plurality of reception antennas.
The MIMO transmission is broadly divided into closed loop type MIMO transmission that requires channel state information (CSI) in a transmission apparatus and open loop type MIMO transmission that does not require the CSI; it is reported that the closed loop type MIMO can achieve more excellent spectrum efficiency than the open loop type MIMO. However, in a radio communication system based on frequency division duplex (FDD) utilizing different carrier wave frequencies in an uplink and a downlink, in order for the base station apparatus to acquire the CSI, it is necessary to feedback the CSI from the mobile station apparatus, there is a problem that overhead is increased significantly.
Hence, in LTE, as disclosed in non-patent document 1, codebook-based closed loop type MIMO transmission is supported that can significantly reduce the amount of overhead needed for notification of the CSI. In the codebook-based closed loop type MIMO, a codebook describing a plurality of linear filters is previously shared between the base station apparatus and the mobile station apparatus, and the mobile station apparatus extracts the desired transmission filter from the codebook and notifies its number (index) to the base station apparatus. The base station apparatus performs, based on the notified linear filter, precoding on transmission data, and thereafter performs MIMO transmission. Since the CSI is notified based on the codebook, as compared with the method in which the mobile station apparatus notifies the CSI itself, it is possible to significantly reduce the amount of overhead. However, in order for the mobile station apparatus to extract the desired transmission filter, it is necessary to calculate the reception quality (such as a reception signal-to-noise power ratio (reception SNR), a reception signal-to-interference plus noise power ratio (reception SINR) or a communication capacity (capacity) etc. for all the linear filters described in the codebook and to compare the results, the burden of the mobile station apparatus is increased.
On the other hand, LTE supports a multiuser MIMO (MU-MIMO) in which a plurality of mobile station apparatuses connected simultaneously are regarded as an imaginary large-scale antenna array, and in which transmission signals from the base station apparatus to the mobile station apparatuses are spatially multiplexed. This is also realized by the codebook-based. As in the SU-MIMO, in the MU-MIMI, the mobile station apparatus notifies the desired linear filter from the codebook to the base station apparatus, and the base station apparatus determines, based on the linear filters notified from a plurality of mobile station apparatuses, whether to perform the MU-MIMO in which data for a plurality of mobile station apparatuses is spatially multiplexed and transmitted.
However, in the codebook-based MU-MIMI in which the mobile station apparatus does not notify the CSI itself, spatial multiplexing opportunity is not significantly enhanced. Hence, non-patent document 2 and the like discuss a technology called Best companion PMI feedback in which the mobile station apparatus notifies the base station apparatus of not only the desired linear filter but also Best companion PMI representing the best compatible linear filter for spatially multiplexing with the desired linear filter. This is a method in which the base station apparatus acquires the desired linear filters notified from a plurality of mobile station apparatuses and the Best companion PMI, and in which the mobile station apparatuses where the desired linear filter in one is the Best companion PMI in another are spatially multiplexed each other; as compared with the pure codebook-based MU-MIMO, it is possible to enhance the spatial multiplexing. However, in order to notify the Best companion PMI, there is a problem that the amount of overhead is increased.
Hence, non-patent document 3 and the like have recently discussed a technology in which a group of linear filters described in the codebook are categorized (grouped or clustered) based on a certain rule to reduce the amount of overhead need for notifying the Best companion PMI. Non-patent document 3 indicates a method in which, a group of linear filters described in the codebook for four transmission antennas adopted in LTE are divided into four clusters to reduce the amount of overhead need for notifying the Best companion PMI. However, non-patent document 3 targets only the codebook for four transmission antennas described in Table 6. 3. 4. 2. 3-2 of non-patent document 4, and does not mention the codebook for 8 transmission antennas that has been determined to be adopted in LTE-A described in non-patent document 5. As compared with the codebook for four transmission antennas, in the codebook for 8 transmission antennas, a significantly large number of linear filters are described, and a large burden is placed on the mobile station apparatus only by extracting the desired linear filter, and thus as compared with the codebook for four transmission antennas, it is important to categorize the codebook.